Diffusing substrate

ABSTRACT

Diffusing substrate ( 20 ) comprising a glass substrate ( 21 ) and a diffusing layer ( 22 ) deposited on the said glass substrate, characterized in that the glass substrate ( 21 ) has a light transmission at least equal to 91% calculated over the 380 to 780 nm wavelength range according to the EN 410 standard.

The present invention relates to a diffusing substrate for making alight source uniform.

The invention will be more particularly described with reference to adiffusing substrate used for making the light emitted by a backlightingsystem uniform.

A backlighting system, which consists of a light source or backlight, isused, for example, as backlighting source for liquid-crystal screens,also called LCD screens. It turns out that the light thus emitted by thebacklighting system is not sufficiently uniform and exhibits overlystrong contrasts. Diffusing means associated with the backlightingsystem are therefore needed to make the light uniform.

Among liquid-crystal screens, a distinction may be made between screensthat incorporate a structure called “direct light”, for which the lightsources are located inside an enclosure and the diffusing means areplaced in front of the light sources, and screens that incorporate astructure called “edge light” for which the light sources are positionedon the side of the enclosure, the light being conveyed to the diffusingmeans at the front face by a waveguide. The invention relates moreparticularly to LCD screens with a direct-light structure.

The invention may also be used when it is desired to make the lightcoming from architectural flat lamps uniform, these lamps being used,for example, on ceilings, floors or walls. They may also be flat lampsfor municipal use, such as lamps for advertising panels or else lampsthat can constitute shelves or bottoms of display windows.

One satisfactory solution from the uniformity stand-point consists incovering the front face of the backlighting system with a sheet ofplastic, such as a polycarbonate or an acrylic polymer bulk-filled withmineral fillers, the sheet having a thickness of 2 mm for example.However, since this material is heat-sensitive, the plastic ages badlyand the heat generated generally results in structural deformation ofthe plastic diffusing means, which is manifested by non-uniformity ofthe luminance of the projected image on the LCD screen for example.

It may therefore be preferred to use, as diffusing means, a diffusinglayer such as that described in French Patent Application publishedunder No. 2 809 496. This diffusing layer composed of agglomeratedparticles in a binder is deposited on a substrate, for example made ofglass.

However, the inventors have shown that the use of such diffusing meanscauses, at the interfaces with the glass substrate, many reflections ofthe light generated by the backlighting system. Furthermore, althoughthe backlighting system possesses reflectors for reflecting the lightreflected by the glass substrate that could not be transmitted, thelight sent back by the reflectors towards the glass substrate is,however, only partly transmitted, a portion being again reflected andsent back once more by the reflectors, and so on. Thus, all the light isnot transmitted immediately the backlighting system is operated, buttravels forwards and backwards several times before passing through thediffusing substrate, with some losses. The inventors have chosen to callthis phenomenon the “recycling” phenomenon.

Having demonstrated this recycling phenomenon, which problem hadhitherto never been eliminated, the inventors have established that itis necessary to study the quality of transmission of the light throughthe diffusing substrate in order to obtain suitable luminance of theillumination emanating from the substrate.

Moreover, the inventors have shown that too thick a glass substrate cangenerate excessive absorption and consequently can generate insufficientluminance, resulting in a lowering of the luminance of the image on anLCD screen for example.

The object of the invention is therefore to provide a diffusingsubstrate that includes a glass substrate coated with a diffusing layerand that makes it possible to optimize the luminance of the illuminationgenerated by means of such a substrate.

According to the invention, to optimize the luminance of theillumination generated by means of the diffusing substrate that includesa glass substrate and a diffusing layer deposited on the said glasssubstrate, the diffusing substrate is characterized in that the glasssubstrate has a light transmission at least equal to 91%, and preferablyat least equal to 91.50%, calculated over the 380 to 780 nm wavelengthrange according to the EN 410 standard, for a glass having an index of1.52±0.04.

The inventors have been able to demonstrate that the luminance, whichdepends on the quality of the light transmission of the substrate,depends on parameters such as the linear absorption coefficient and thethickness of the glass substrate, the linear absorption coefficientbeing tied to the glass composition of the substrate.

Thus, according to one feature, the glass substrate has a total ironcontent such that:$\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t} \leq \frac{7110}{\left( {{1.52 \times e} + 0.015} \right) + {\left( {{17.24 \times e} + 0.37} \right) \times {redox}}}$with [Fe₂O₃]_(t) expressed in ppm and corresponding to the total iron inthe composition, e being the thickness of the glass in mm and the redoxbeing defined by redox=[FeO]/[Fe₂O₃]_(t), the redox being between 0 and0.9.

According to another feature, the iron content must be even furtherlimited if the light transmission is at least equal to 91.50%. Thiscontent is then such that:$\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t} \leq \frac{2110}{\left( {{1.52 \times e} + 0.015} \right) + {\left( {{17.24 \times e} + 0.37} \right) \times {redox}}}$with [Fe₂O₃]_(t) expressed in ppm and corresponding to the total iron inthe composition, e being the thickness of the glass in mm and the redoxbeing defined by redox=[FeO]/[Fe₂O₃]_(t), the redox being between 0 and0.9.

Also, according to a first embodiment, the glass substrate has a minimumlight transmission of 91.50% for a thickness e of at most 4.0 mm, with atotal iron content of 200 ppm and a redox of less than 0.05.

According to a second embodiment, the glass substrate has a minimumlight transmission of 91% for a thickness e of at most 4.0 mm, with atotal iron content of 160 ppm and a redox of 0.31. For the same ironcontent and redox, the thickness e will be at most 1.5 mm in order toensure the 91.50% minimum light transmission property.

Again, according to a third embodiment, the glass substrate has aminimum light transmission of 91% for a thickness e of at most 1.2 mm,with a total iron content of 800 ppm and a redox of 0.33.

According to yet another embodiment, the glass substrate has a minimumlight transmission of 91% for a thickness e of at most 1.2 mm, with atotal iron content of 1050 ppm and a redox of 0.23.

According to one feature, the glass composition of the glass substrateof the invention comprises at least the following constituents: % byweight SiO₂ 65-75 Al₂O₃ 0-5 CaO  5-15 MgO  0-10 Na₂O  5-20 K₂O  0-10 BaO0-5 ZnO 0-5

According to another feature, the diffusing layer of the substrate ofthe invention is composed of agglomerated particles in a binder, thesaid particles having a mean diameter of between 0.3 and 2 microns, thesaid binder being in a proportion of between 10 and 40% by volume andthe particles forming aggregates whose size is between 0.5 and 5microns. The particles are semi-transparent particles and preferablymineral particles, such as oxides, nitrides and carbides. The particlesare preferably chosen from silicon, aluminium, zirconium, titanium andcerium oxides, or a mixture of at least two of these oxides. For furtherdetails, reference may be made to the published application FR 2 809496.

Finally according to the invention, this diffusing substrate will inparticular be used in a backlighting system that can be provided in anLCD screen or in a flat lamp.

Other advantages and features of the invention will become apparent inthe rest of the description in conjunction with the appended drawings inwhich:

FIG. 1 illustrates a backlighting system;

FIG. 2 illustrates curves giving, for a 91% light transmission, thetotal iron Fe₂O₃ content as a function of the redox for several glassthicknesses,

FIG. 3 illustrates curves giving, for a 91.5% light transmission, thetotal iron Fe₂O₃ content as a function of the redox for several glassthicknesses.

For the sake of clarity, various elements have not been drawn to scale.

FIG. 1 illustrates a backlighting system 1, intended for example to beused in an LCD screen with a size of 17″ for example. The system 1comprises an enclosure 10, that includes an illuminant or light sources11, and a glass diffusing substrate 20 that is joined to the enclosure10.

The enclosure 10, with a thickness of about 10 mm, has a lower part 12in which the light sources 11 are provided and, opposite it, an upperpart 13 which is open and from which the light emitted by the sources 11propagates. The lower part 12 has a bottom 14 against which there arereflectors 15 for reflecting, on the one hand, a portion of the lightemitted by the sources 11 that is directed towards the lower part 12and, on the other hand, a portion of the light that is not transmittedthrough the diffusing substrate but reflected by the glass substrate andbackscattered by the diffusing layer. The arrows shown illustrateschematically the paths of the light emitted by the sources 11 andrecycled in the enclosure.

The light sources 11 are, for example, discharge lamps or tubes, usuallycalled CCFLs “Cold Cathode Fluorescent Lamps”, HCFLs “Hot CathodeFluorescent Lamps” or DBDFLs “Dielectric Barrier Discharge FluorescentLamps”, or else lamps of the LED “Light Emitting Diode” type.

The diffusing substrate 20 is attached to the upper part 13 and heldfast by mechanical fastening means (not illustrated) such as clipscooperating with the enclosure and the substrate, or else held in placeby mutual engagement means (not illustrated) such as a groove providedon the periphery of the surface of the substrate cooperating with aperipheral rib on the enclosure.

The diffusing substrate 20 comprises a glass substrate 21 and adiffusing layer 22, with a thickness of between 1 and 20 μm, placed onone face of the glass substrate, facing or opposite the upper part 13 ofthe enclosure. For the composition of the layer and its deposition onthe glass substrate, reference may be made to French Patent Applicationpublished under 2 809 496.

The substrate 21 for supporting the layer is made of glass that istransparent or semi-transparent in the visible wavelength range. It ischaracterized according to the invention by its low light absorption andhas a light transmission T_(L) of least 91% over the 380 to 780 nmwavelength range. The light transmission is calculated under illuminantD₆₅ according to the EN410 standard.

Given below in the form of a table are illustrative examples of theglass substrate 21, the table indicating, for each of them, the glasscomposition, the contents of which are expressed in % by weight, thetotal iron content, the ferrous iron content, the redox and the lighttransmission T_(L) under illuminant D₆₅.

The light transmission T_(L) is calculated for a given thickness e ofthe glass substrate. Examples 1a, 1b, 2 and 3 are glass substrates thatmeet the at least 91% light transmission property, whereas Example 4does not. These examples are substrates made of commercially availableglass sold under the following names:

-   -   Example 1a: B270 from Schott, where e=0.9 mm;    -   Example 1b: B270 from Schott, where e=2.0 mm (in Examples 1a and        1b, only the thicknesses differ, the glass composition being        identical);    -   Example 2: OPTIWHITE from Pilkington, where e=1.8 mm;    -   Example 3: CS77 from Saint-Gobain Glass, where e=1.1 mm; and

Example 4: PLANILUX from Saint-Gobain Glass, where e=2.1 mm. Example 1aand Example 1b Example 2 Example 3 Example 4 SiO₂ 69.84 71.81 69 71.12Al₂O₃ 0.08 0.6 0.5 0.5 CaO 6.8 8.9 10 9.45 MgO 0.15 4.4 0 4.4 MnO 0 0 00.002 Na₂O 8.15 13.55 4.5 13.8 K₂O 8.5 0.4 5.5 0.25 BaO 1.8 0 0 0 TiO₂0.2 0.02 0 0.02 Sb₂O₃ 0.45 0 0 0 SrO 0 0 7 0 ZnO 3.6 0.001 0 0 ZrO₂ 00.01 3.5 0 Fe₂0₃ in 200 160 800 1050 ppm FeO in <10 50 260 240 ppm Redox<0.05 0.31 0.33 0.23 T_(L) in % 91.58 (e = 0.9 mm) 91.4 91.0 90.6 91.51(e = 2.0 mm) (e = 1.8 mm) (e = 1.1 mm) (e = 2.1 mm)

It should be noted that these compositions have impurities, the natureand the proportions of which are, for some of them, summarized below:Cr₂O₃<10 ppm;MnO<300 ppm;V₂O₅<30 ppm;TiO₂<1000 ppm.

The light transmission T_(L) is calculated over the 380-780 nmwavelength range according to the EN 410 standard on the basis of thetransmission τ that is defined in a known manner by the Beer-LambertLaw:τ(λ)≠(1−R(λ))² e ^(−α(λ)e)where:

-   -   R is the reflection factor;    -   α is the linear absorption coefficient (α and R depending on the        wavelength of the light emitted); and    -   e is the thickness of the substrate.

The light transmission T_(L) therefore depends on the linear absorptioncoefficient a and the thickness e of the substrate 21.

The inventors have consequently demonstrated that the glass compositionof the substrate and its thickness have an influence on the lighttransmission of the substrate. More particularly, the total iron content(expressed as Fe₂O₃) and the redox of the composition play a major roleas regards the linear absorption coefficient. In the invention, theredox is defined as being the ratio of the content of iron in reducedform (expressed as FeO) to the total iron content (expressed as Fe₂O₃),namely the FeO/Fe₂O₃ ratio.

Thus, the thickness of the substrate may be selected according to theglass composition used.

The inventors have established a relationship between the parameters,that is to say the thickness of the glass, the total iron and the redoxof the glass composition that result in the required light transmissionproperty. This constraint relationship may be written in the followingmathematical form—the total iron content in the composition is suchthat, for a light transmission T_(L) greater than or equal to 91%:$\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t} \leq \frac{7110}{\left( {{1.52 \times e} + 0.015} \right) + {\left( {{17.24 \times e} + 0.37} \right) \times {redox}}}$with [Fe₂O₃]_(t) expressed in ppm and corresponding to the total iron inthe composition, e being the thickness of the glass in mm and theredox=[FeO]/[Fe₂O₃]_(t), the redox being between 0 and 0.9.

As a variant, the constraint may be placed on the thickness for a givenglass composition and is such that, for a light transmission T_(L) ofgreater than or equal to 91%:$e \leq {\frac{{7110/\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t}} - 0.015 - {0.37 \times {redox}}}{1.52 + {17.24 \times {redox}}}.}$

For a light transmission T_(L) of 91.5%, which is a preferred minimumvalue according to the invention, the total iron content in thecomposition must be even lower than that expressed above in the case ofa lower transmission limit of 91%, and is such that:$\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t} \leq \frac{2110}{\left( {{1.52 \times e} + 0.015} \right) + {\left( {{17.24 \times e} + 0.37} \right) \times {redox}}}$or the thickness must be such that:$e \leq {\frac{{2110/\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t}} - 0.015 - {0.37 \times {redox}}}{1.52 + {17.24 \times {redox}}}.}$

The inequalities given above, linking the values of the Fe₂O₃/redox pairand the thickness of the substrate, may be expressed in the form ofcurves for characteristic glass thicknesses.

Thus, FIG. 2 illustrates curves giving, for various given thicknessesrespectively, the total iron content Fe₂O₃ as a function of the redoxfor a light transmission T_(L) of 91%. The substrates of definedthickness, the iron and redox values of the glass composition of whichlie on or below the reference curve for the same chosen thickness, aresuitable for meeting the light transmission property of having to be atleast 91%.

Plotted in this figure are the points EX1, EX2, EX3 and EX4 of theFe₂O₃/redox pair of the glass composition corresponding to Examples 1aand 1b in the case of the point EX1 and to Examples 2, 3 and 4 for theother points, EX2, EX3 and EX4 respectively.

It should be noted that the point EX1 lies well below the 2.1 mm curveand even below the 4 mm curve. Consequently, the glass substrate ofExamples 1a and 1b is suitable with a thickness of 0.9 mm and 2.0 mmrespectively, and the glass composition could even be suitable with ahigher thickness, up to 4 mm at least, in order to have a 91% minimumlight transmission. However, it is not of interest when constructing thebacklighting system to increase the thickness of the elements, as thecurrent trend is towards a reduction in the size of LCD screens in termsof thickness. Therefore a thickness of more than 4 mm will not beenvisaged.

The same comment applies to the point EX2, which is well below the curvecorresponding to the 1.8 mm thickness of the substrate of Example 2. Theglass composition of Example 2 would be suitable for a substrate with athickness not exceeding 4.0 mm in order to have a 91% minimum lighttransmission.

It should also be noted that the point EX3 is below the 1.1 mm curvecorresponding to the thickness of Example 3. However, with a thicknessof more than 1.2 mm (curves below this point), the glass composition ofExample 3 would no longer be suitable for achieving a 91% minimumtransmission.

In contrast, the point EX4 is well above the 2.1 mm thickness curvecorresponding to Example 4, which therefore is not suitable. However, itmay be deduced therefrom that, by reducing the thickness of this type ofglass so that it has a thickness of less than 1.2 mm at least (curvesabove this point), this glass composition would be suitable forobtaining the 91% light transmission property.

FIG. 3 illustrates curves giving, for several given thicknessesrespectively, the total iron content Fe₂O₃ as a function of the redoxfor a minimum light transmission T_(L) of 91.50%.

This shows that, for a 91.50% light transmission, which constitutes apreferred minimum value of the invention, only Examples 1a and 1b, thepoint EX1 of which lies well below the curve corresponding to the 2.1 mmthickness, are suitable. The other examples are not suitable forachieving a light transmission of 91.50% at least, since the points EX2,EX3 and EX4 lie above the curves corresponding to the respectivethicknesses of Examples 2, 3 and 4. It may be noted that the point EX2is substantially above the curve corresponding to the 1.8 mm thicknessand that it would be suitable in the case of the glass composition ofExample 2 to produce a thinner substrate, for example with a thicknessof 1.5 mm (which corresponds to the first curve lying above the point)so as to achieve the minimum 91.50% light transmission property.

The glass substrate 21 is therefore used as a support for the diffusinglayer 22 so as to constitute the diffusing substrate 20 that isassociated with the enclosure 10 in order to constitute the backlightingsystem 1. It is then possible to measure in a known manner the luminanceof the illumination emanating from the enclosure and passing through thediffusing substrate. The table below summarizes, for Examples 1a, 1b and2 to 4, the luminance associated with the light transmission. The valuesof the luminance given correspond to a measurement made perpendicular tothe surface of the diffusing substrate and for a diffusing substrate(glass substrate and diffusing layer) having a diffuse transmission of60%, that is to say 40% of the light is backscattered by the diffusingsubstrate, which backscattered light is recycled within the enclosure.Example Example 1a 1b Example 2 Example 3 Example 4 T_(L) in % 91.5891.51 91.4 91.0 90.6 Luminance 3997 3983 3965 3956 3811 in cd/m²

Moreover, the glass substrate also has the advantage of serving as asupport for depositing functional multi-layer coatings such as anelectromagnetic insulation coating that may also constitute thediffusing layer 22 as described in French Patent Application FR02/08289, or a coating with a low-emissivity function, an antistatic,antifogging or antisoiling function, or else a luminance-increasingfunction. This latter function may actually be desirable when thediffusing substrate is applied to an LCD screen.

A coating having the function of further increasing the luminance bytightening the scattering indicatrix is, for example, known in the formof an optical film sold under the name CH27 by SKC.

The table below indicates, in addition to the light transmission for theglass substrate 21, the lumination luminances obtained without the CH27coating and with the CH27 coating on the diffusing substrate 20, and theratio of these two luminances are expressed in %. The given values ofthe luminance correspond to a measurement made perpendicular to thesurface of the diffusing substrate and for a diffusing substrate (glasssubstrate and diffusing layer) having a diffuse transmission of 60%.Without T_(L) in % CH27 With CH27 Ratio in % Example 1a 91.58 3997 556028.10 Example 1b 91.51 3983 5489 27.43 Example 2 91.4 3965 5417 26.80Example 3 91.0 3956 5303 25.40 Example 4 90.6 3811 4994 23.68

Of course, it should be noted that the luminance increases with CH27—itis the function of the latter—but also that the increase in luminance ismuch higher when the light transmission is higher. These results showthe benefit of using a substrate 21 made of the least absorbent glasspossible, in order to optimize the luminance of a backlighting system.In this regard, the substrate of Example 1a or 1b will be preferred.

1. Diffusing substrate (20) comprising a glass substrate (21) and adiffusing layer (22) deposited on the said glass substrate,characterized in that the glass substrate (21) has a light transmissionat least equal to 91% calculated over the 380 to 780 nm wavelength rangeaccording to the EN 410 standard.
 2. Diffusing substrate according toclaim 1, characterized in that the light transmission is at least equalto 91.5%.
 3. Diffusing substrate according to claim 1, characterized inthat the glass substrate (21) has a total iron content such that:$\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t} \leq \frac{7110}{\left( {{1.52 \times e} + 0.015} \right) + {\left( {{17.24 \times e} + 0.37} \right) \times {redox}}}$with [Fe₂O₃]_(t) expressed in ppm and corresponding to the total iron inthe composition, e being the thickness of the glass in mm and the redoxbeing defined by redox=[FeO]/[Fe₂O₃]_(t), the redox being between 0 and0.9.
 4. Diffusing substrate according to claim 2, characterized in thatthe glass substrate (21) has a total iron content such that:$\left\lbrack {{Fe}_{2}O_{3}} \right\rbrack_{t} \leq \frac{2110}{\left( {{1.52 \times e} + 0.015} \right) + {\left( {{17.24 \times e} + 0.37} \right) \times {redox}}}$with [Fe₂O₃]_(t) expressed in ppm and corresponding to the total iron inthe composition, e being the thickness of the glass in mm and the redoxbeing defined by redox=[FeO]/[Fe₂O₃]_(t), the redox being between 0 and0.9.
 5. Diffusing substrate according to any one of the precedingclaims, characterized in that the diffusing layer (22) is composed ofagglomerated particles in a binder, the said particles having a meandiameter of between 0.3 and 2 microns, the said binder being in aproportion of between 10 and 40% by volume and the particles formingaggregates whose size is between 0.5 and 5 microns.
 6. Diffusingsubstrate according to claim 5, characterized in that the particles aresemi-transparent particles and preferably mineral particles, such asoxides, nitrides and carbides.
 7. Diffusing substrate according to anyone of the preceding claims, characterized in that the glass substrate(21) has a glass composition based on at least the followingconstituents: % by weight SiO₂ 65-75  Al₂O₃ 0-5  CaO 5-15 MgO 0-10 Na₂O5-20 K₂O 0-10 BaO 0-5  ZnO 0-5 


8. Diffusing substrate according to claim 1 or 2, characterized in thatthe glass substrate (21) has a minimum light transmission of 91.50% fora thickness e of at most 4.0 mm, with a total iron content of 200 ppmand a redox of less than 0.05.
 9. Diffusing substrate according to claim1, characterized in that the glass substrate (21) has a minimum lighttransmission of 91% for a thickness e of at most 4.0 mm, with a totaliron content of 160 ppm and a redox of 0.31.
 10. Diffusing substrateaccording to claim 2, characterized in that the glass substrate (21) hasa minimum light transmission of 91.50% for a thickness e of at most 1.5mm, with a total iron content of 160 ppm and a redox of 0.31. 11.Diffusing substrate according to claim 1, characterized in that theglass substrate (21) has a minimum light transmission of 91% for athickness e of at most 1.2 mm, with a total iron content of 800 ppm anda redox of 0.33.
 12. Diffusing substrate according to claim 1,characterized in that the glass substrate (21) has a minimum lighttransmission of 91% for a thickness e of at most 1.2 mm, with a totaliron content of 1050 ppm and a redox of 0.23.
 13. Use of a diffusingsubstrate as described in one of claims 1 to 12 for producing abacklighting system.
 14. Use according to claim 13, for which thebacklighting system is provided in an LCD screen.
 15. Use according toclaim 13, for which the backlighting system is provided in a flat lamp.